The present invention relates to semiconductor devices, and, more particularly, to semiconductor materials for the layers therein. The purposes of particular layers can be directed towards such goals as improved ohmic contact between these devices and external circuits, improved electroluminescent, or devices having improved reliability.
Electroluminescent devices, for example, are usually constructed of layers of single crystalline semiconductor materials. When biased, these devices emit electromagnetic energy through the process of recombination of oppositely charged carriers. Typical devices are light emitting diodes and lasers having a dual heterojunction.
Because these devices are made of layers of different semiconductor materials, problems arise naturally in the fabrication of these materials into different layers, such as lattice mismatch between the layers, and different thermal coefficients of expansion. Another problem in manufacturing semiconductor devices lies in attaching metal electrical leads to the semiconductor material. Metal to semiconductor contacts, commonly called ohmic contacts, must not add significant resistance to the structure and should not alter the equilibrium carrier densities within the semiconductor so as to change the characteristics of the device. Some metal to semiconductor contacts may form rectifying barriers which render them unacceptable for use in this area. Often, a highly doped or degenerate semiconductor region is employed to act as an interface between the metal and semiconductor material. This region is doped so that the two resulting contacts, a metal-semiconductor and a semiconductor-semiconductor contact, have relatively linear resistivity and other desirable properties.
Satisfactory ohmic contacts are particularly difficult to form on P type semiconductor material having an energy bandgap in excess of 1.6 electron volts (ev). Under these circumstances, the degenerate semiconductor region usually has a narrow energy bandgap than the adjacent semiconductor material thus providing a better surface to which the metal contact layer can be applied. However, this creates additional problems with respect to the matching of the crystal lattices of the wide energy bandgap material to the degenerate region. If the lattices of the two semiconductor regions differ substantially, stresses will occur at the interface between the two regions. These stresses may result in a loss of efficiency in the semiconductor and may even cause the semiconductor material to crack at the semiconductor interface thereby physically damaging the device.
In particular, when InGaAsP is used as the degenerate semiconductor layer for ohmic contact or is used as the active layer for a laser device, lattice mismatch between InGaAsP and, for example, InP causes at times the InGaAsP layer to physically shear-off during mounting or to separate from the underlying structure during device operation. Further, carryover of expitaxial solutions onto the surface of the InGaAsP layer on growth completion lessens the amount of useable material for device fabrication.
Minimizing lattice mismatch between the different layers, selecting layers with similar thermal coefficients of expansion, and minimizing fabrication problems would clearly lead to improved electroluminescent devices.